Ultra-low-current driven InGaN blue micro light-emitting diodes for electrically efficient and self-heating relaxed microdisplay

InGaN-based micro-light-emitting diodes have a strong potential as a crucial building block for next-generation displays. However, small-size pixels suffer from efficiency degradations, which increase the power consumption of the display. We demonstrate strategies for epitaxial structure engineering carefully considering the quantum barrier layer and electron blocking layer to alleviate efficiency degradations in low current injection regime by reducing the lateral diffusion of injected carriers via reducing the tunneling rate of electrons through the barrier layer and balanced carrier injection. As a result, the fabricated micro-light-emitting diodes show a high external quantum efficiency of 3.00% at 0.1 A/cm2 for the pixel size of 10 × 10 μm2 and a negligible Jmax EQE shift during size reduction, which is challenging due to the non-radiative recombination at the sidewall. Furthermore, we verify that our epitaxy strategies can result in the relaxation of self-heating of the micro-light-emitting diodes, where the average pixel temperature was effectively reduced.

A single conduction band diagram was numerically calculated and compared at 0.1 A/cm 2 , and the 2 nd quantum-well from the n-doped GaN was aligned in the same position to compare the energy barrier caused by different polarization from QB thickness. The magnitude of the slope indicates the strength of the electric field in the QW and the QB 1 , and due to the conservation of the electrostatic field, the net charge is zero, which can be described in following equation 2 : (1) where dQW and dQB are the thickness of QW and QB, and EQW and EQB are the electric fields in QW and QB. When the thickness of QW is fixed, the electric field in the QW increases with increasing QB thickness, which increases the conduction band slope inside the QW. However, as depicted in Supplementary Fig. 1, the energy height of QB increases simultaneously, which is one of the factors that influence the carrier confining capability of the QW. Supplementary Fig. 3 shows the J-V characteristics of differently sized μLEDs from 80 × 80 μm 2 to 10 × 10 μm 2 . The current density in the low forward bias range between 1.5 V and 2.7 V seems overlapping when plotting the J-V characteristics of different sizes. However, a gradual increase of current density was observed with decreasing size of the μLEDs when the current density was magnified, and was found the increase rate of QB 3.5 was higher than that of QB 7.5, and QB 7.5 was slightly higher. Specifically, in the case of QB 3.5, the current density at the bias of 2.4 V increase 2468.7 % when the pitch size decreased from 80 × 80 μm 2 to 10 × 10 μm 2 , while 140. 6%,139.9%, and 37.1% increase in current density was observed in the case of QB 7.5, QB 10.5, and QB 10.5 Balanced EBL, respectively.  Equation (2) The supplementary equation (2)       Supplementary Fig. 9 shows the EQE-logarithmic current density curve for the sidewall passivated devices of QB 3.5, QB 7.5, QB 10.5, and QB 10.5 Balanced EBL, and it is summarized and compared with the devices without sidewall passivation in Supplementary Fig.   10. From the figure, we observed enhancements both in maximum EQE and Jmax EQE. This is because of the decrease in the surface current as shown in Supplementary Fig. 8. Furthermore, the increasing EQE trend of the epitaxial structure from QB 3.5 to QB 10.5 Balanced EBL is also still valid after the sidewall passivation, which further convinces that the optimization of   Supplementary Fig. 11b. This increased accumulation of electrons and leakage current can increase the lateral diffusion of electrons to the sidewall in the low current region.
However, through the simulation, we found that the importance of the spacer in QB 10.5 Balanced EBL is not as significant as in QB 10.5. Supplementary Fig. 11c shows the energy band diagram QB 10.5 Balanced EBL and QB 10.5 Balanced EBL w/ spacer. Due to the lowered Al composition and induced higher p-type doping concentration of EBL in QB 10.5 Balanced EBL, the energy dip is not formed in the interface between the last QB and EBL in the energy band diagram, which means the necessity to compensate the electric field between this interface is not significant. The simulated leakage current results in Supplementary Fig.   11d also show that there is no significant difference between the leakage current.  Fig. 12 Average temperature of devices fabricated from QB 3.5, QB 7.5, QB 10.5, and QB 10.5 Balanced EBL (BE) with and without sidewall passivation at different sizes and LOP per area. The X corresponds to devices without passivation and O corresponds to devices with sidewall passivation. The pitch size presented in the figure showing 80 μm and 10 μm corresponds to 80 × 80 μm 2 and 10 × 10 μm 2 , respectively.

Supplementary
Supplementary Table 2 shows the thermoreflectance microscopy image of QB 3.5 to QB 10.5 Balanced EBL both for devices with and without sidewall passivation with different light output power (LOP) per area at different pitch sizes of 10 μm 2 × 10 μm 2 and 80 μm 2 × 80 μm 2 .
The resulting average temperature is summarized in Supplementary Fig. 12. The resulting average temperature shows a trend that the sidewall passivated devices with lower average temperature than the devices without sidewall passivation both for 10 μm 2 × 10 μm 2 and 80 μm 2 × 80 μm 2 sized devices, especially at higher LOP per area of 1 W/cm 2 . In the meanwhile, at a lower LOP per area of 0.1 W/cm 2 , the average temperature has negligible change after the sidewall passivation, which emphasizes the importance of the necessity of low-current operation in μLEDs display. The thermoreflectance microscopy (TRM) setup schematic is shown in Supplementary Fig. 14.
The broadband light source is passed through a bandpass filter which eliminates the emission wavelength of LED samples. The light is then reflected from the LED sample, and detected by the CCD. Since reflectivity is related to refractive index, and refractive index is influenced by temperature, it is possible to measure the reflectivity change of the sample surface in terms of temperature by controlling the temperature using a thermoelectric cooler element.
Supplementary equation (5) describes the approximated relation between the change of reflectivity and the change of temperature in first-order equation 10 .
Equation (5) where R is reflectivity, T is temperature, and κ is the thermoreflectance coefficient. By linear fitting the relation ∆R/R and ∆T, κ can be extracted as shown in Supplementary Fig. 15.